Demand-based cardiac function therapy

ABSTRACT

A method and device for delivering cardiac function therapy on a demand basis. An implantable device for delivering cardiac function therapy is programmed to suspend such therapy at periodic intervals or upon command from an external programmer. Measurements related to hemodynamic performance are then taken using one or more sensing modalities incorporated into the device. Based upon these measurements, the device uses a decision algorithm to determine whether further delivery of the cardiac function therapy is warranted.

FIELD OF THE INVENTION

This patent application pertains to methods and apparatus for thetreatment of cardiac disease. In particular, it relates to methods andapparatus for improving cardiac function with electro-stimulatorytherapy.

BACKGROUND

Implantable cardiac devices that provide electrical stimulation toselected chambers of the heart have been developed in order to treat anumber of cardiac disorders. A pacemaker, for example, is a device whichpaces the heart with timed pacing pulses, most commonly for thetreatment of bradycardia where the ventricular rate is too slow.Atrio-ventricular conduction defects (i.e., AV block) and sick sinussyndrome represent the most common causes of bradycardia for whichpermanent pacing may be indicated. If functioning properly, thepacemaker makes up for the heart's inability to pace itself at anappropriate rhythm in order to meet metabolic demand by enforcing aminimum heart rate. Implantable devices may also be used to treatcardiac rhythms that are too fast, with either anti-tachycardia pacingor the delivery of electrical shocks to terminate atrial or ventricularfibrillation.

Implantable devices have also been developed that affect the manner anddegree to which the heart chambers contract during a cardiac cycle inorder to promote the efficient pumping of blood. The heart pumps moreeffectively when the chambers contract in a coordinated manner, a resultnormally provided by the specialized conduction pathways in both theatria and the ventricles that enable the rapid conduction of excitation(i.e., depolarization) throughout the myocardium. These pathways conductexcitatory impulses from the sino-atrial node to the atrial myocardium,to the atrio-ventricular node, and thence to the ventricular myocardiumto result in a coordinated contraction of both atria and bothventricles. This both synchronizes the contractions of the muscle fibersof each chamber and synchronizes the contraction of each atrium orventricle with the contralateral atrium or ventricle. Without thesynchronization afforded by the normally functioning specializedconduction pathways, the heart's pumping efficiency is greatlydiminished. Pathology of these conduction pathways and otherinter-ventricular or intra-ventricular conduction deficits can be acausative factor in heart failure, which refers to a clinical syndromein which an abnormality of cardiac function causes cardiac output tofall below a level adequate to meet the metabolic demand of peripheraltissues. In order to treat these problems, implantable cardiac deviceshave been developed that provide appropriately timed electricalstimulation to one or more heart chambers in an attempt to improve thecoordination of atrial and/or ventricular contractions, termed cardiacresynchronization therapy (CRT). Ventricular resynchronization is usefulin treating heart failure because, although not directly inotropic,resynchronization can result in a more coordinated contraction of theventricles with improved pumping efficiency and increased cardiacoutput. Currently, a most common form of CRT applies stimulation pulsesto both ventricles, either simultaneously or separated by a specifiedbiventricular offset interval, and after a specified atrio-ventriculardelay interval with respect to the detection an intrinsic atrialcontraction.

Cardiac pacing therapy, if delivered synchronously, is demand-based.That is, pacing pulses are delivered only when the heart's intrinsicrhythm fails to maintain an adequate heart rate. Cardiacelectro-stimulation delivered for purposes other than to enforce aminimum rate, however, is currently delivered in a more or less constantmanner without regard for changes in the patient's condition.

SUMMARY

The present invention relates to a method and device for deliveringcardiac function therapy on a demand basis. In accordance with theinvention, an implantable device for delivering cardiac function therapyis programmed to suspend such therapy at periodic intervals or uponcommand from an external programmer. Measurements related to hemodynamicperformance are then taken using one or more sensing modalitiesincorporated into the device. Based upon these measurements, the deviceuses a decision algorithm to determine whether further delivery of thecardiac function therapy is warranted.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system diagram of a cardiac device configured for multi-sitestimulation and sensing.

FIG. 2 is a block diagram of exemplary components for computing theLF/HF ratio.

FIG. 3 illustrates an exemplary algorithm for implementing theinvention.

DETAILED DESCRIPTION

As noted above, most current cardiac pacing devices are demand based,that is, any pacing mode or pacemaker that delivers an output pulse onlywhen the intrinsic rate is less than the programmed base rate. Thus, ademand interval specifies the time period between two consecutive pacedevents in the same chamber without an intervening sensed event. Asdescribed below, implantable devices for delivering cardiac functiontherapies may be prescribed for post-MI patients or heart failurepatients in order to boost cardiac output and/or to reverse cardiacremodeling. In such cases, the cardiac function therapy delivery can bemade available on a demand basis in accordance with the presentinvention.

1. Cardiac Function Therapy

One example of electro-stimulatory therapy for the purpose of improvingcardiac function is CRT. In ventricular resynchronization therapy, theventricles are paced at more than one site in order to affect a spreadof excitation that results in a more coordinated contraction and therebyovercome interventricular or intraventricular conduction defects.Biventricular pacing is one example of resynchronization therapy inwhich both ventricles are paced in order to synchronize their respectivecontractions. Resynchronization therapy may also involve multi-sitepacing applied to only one chamber. For example, a ventricle may bepaced at multiple sites with excitatory stimulation pulses in order toproduce multiple waves of depolarization that emanate from the pacingsites. This may produce a more coordinated contraction of the ventricleand thereby compensate for intraventricular conduction defects that mayexist.

Another type of cardiac function therapy is stress reduction pacingwhich involves altering the coordination of ventricular contractionswith multi-site pacing in order to change the distribution of wallstress experienced by the ventricle during the cardiac pumping cycle.The degree to which a heart muscle fiber is stretched before itcontracts is termed the preload. The maximum tension and velocity ofshortening of a muscle fiber increases with increasing preload. Theincrease in contractile response of the heart with increasing preload isknown as the Frank-Starling principle. When a myocardial regioncontracts late relative to other regions, the contraction of thoseopposing regions stretches the later contracting region and increasesthe preload. The degree of tension or stress on a heart muscle fiber asit contracts is termed the afterload. Because pressure within theventricles rises rapidly from a diastolic to a systolic value as bloodis pumped out into the aorta and pulmonary arteries, the part of theventricle that first contracts due to an excitatory stimulation pulsedoes so against a lower afterload than does a part of the ventriclecontracting later. Thus a myocardial region that contracts later thanother regions is subjected to both an increased preload and afterload.This situation is created frequently by the ventricular conductiondelays associated with heart failure and ventricular dysfunction. Theheart's initial physiological response to the uneven stress resultingfrom an increased preload and afterload is compensatory hypertrophy inthose later contracting regions of the myocardium. In the later stagesof remodeling, the regions may undergo atrophic changes with wallthinning due to the increased stress. The parts of the myocardium thatcontract earlier in the cycle, on the other hand, are subjected to lessstress and are less likely to undergo hypertrophic remodeling. Thisphenomena may be used to effect reversal of remodeling by pacing one ormore sites in a ventricle (or an atrium) with one or more excitatorystimulation pulses during a cardiac cycle with a specified pulse outputsequence. The pace or paces are delivered in a manner that excites apreviously stressed and remodeled region of the myocardium earlierduring systole so that it experiences less afterload and preload. Thispre-excitation of the remodeled region relative to other regions unloadsthe region from mechanical stress and allows reversal of remodeling tooccur.

2. Hardware Platform

An implantable cardiac device is typically placed subcutaneously orsubmuscularly in a patient's chest with leads threaded intravenouslyinto the heart to connect the device to electrodes used for sensing andstimulation. Leads may also be positioned on the epicardium by variousmeans. A programmable electronic controller causes the stimulus pulsesto be output in response to lapsed time intervals and sensed electricalactivity (i.e., intrinsic heart beats not as a result of a stimuluspulse). The device senses intrinsic cardiac electrical activity by meansof internal electrodes disposed near the chamber to be sensed. Adepolarization wave associated with an intrinsic contraction of theatria or ventricles that is detected by the device is referred to as anatrial sense or ventricular sense, respectively. In order to cause sucha contraction in the absence of an intrinsic beat, a stimulus pulse(a.k.a. a pace or pacing pulse when delivered in order to enforce acertain rhythm) with energy above a certain threshold is delivered tothe chamber.

FIG. 1 shows a system diagram of a microprocessor-based cardiac devicesuitable for practicing the present invention. The device is equippedwith multiple sensing and pacing channels which may be physicallyconfigured to sense and/or pace multiple sites in the atria or theventricles. The device shown in FIG. 1 can be configured for cardiacresynchronization pacing of the atria or ventricles and/or formyocardial stress reduction pacing such that one or more cardiac sitesare sensed and/or paced in a manner that pre-excites at least one regionof the myocardium. The multiple sensing/stimulation channels may beconfigured, for example, with one atrial and two ventricularsensing/stimulation channels for delivering biventricularresynchronization therapy, with the atrial sensing/stimulation channelused to deliver biventricular resynchronization therapy in an atrialtracking mode as well as to pace the atria if required. The controller10 of the pacemaker is a microprocessor which communicates with a memory12 via a bidirectional data bus. The memory 12 typically comprises a ROM(read-only memory) for program storage and a RAM (random-access memory)for data storage. The controller could be implemented by other types oflogic circuitry (e.g., discrete components or programmable logic arrays)using a state machine type of design, but a microprocessor-based systemis preferable. As used herein, the term “circuitry” should be taken torefer to either discrete logic circuitry or to the programming of amicroprocessor.

Shown in the figure are four exemplary sensing and pacing channelsdesignated “a” through “d” comprising bipolar leads with ring electrodes34 a-d and tip electrodes 33 a-d, sensing amplifiers 31 a-d, pulsegenerators 32 a-d, and channel interfaces 30 a-d. Each channel thusincludes a pacing channel made up of the pulse generator connected tothe electrode and a sensing channel made up of the sense amplifierconnected to the electrode. The channel interfaces 30 a-d communicatebidirectionally with microprocessor 10, and each interface may includeanalog-to-digital converters for digitizing sensing signal inputs fromthe sensing amplifiers and registers that can be written to by themicroprocessor in order to output pacing pulses, change the pacing pulseamplitude, and adjust the gain and threshold values for the sensingamplifiers. The sensing circuitry of the pacemaker detects a chambersense, either an atrial sense or ventricular sense, when an electrogramsignal (i.e., a voltage sensed by an electrode representing cardiacelectrical activity) generated by a particular channel exceeds aspecified detection threshold. Pacing algorithms used in particularpacing modes employ such senses to trigger or inhibit pacing, and theintrinsic atrial and/or ventricular rates can be detected by measuringthe time intervals between atrial and ventricular senses, respectively.

The electrodes of each bipolar lead are connected via conductors withinthe lead to a MOS switching network 70 controlled by the microprocessor.The switching network is used to switch the electrodes to the input of asense amplifier in order to detect intrinsic cardiac activity and to theoutput of a pulse generator in order to deliver a pacing pulse. Theswitching network also enables the device to sense or pace either in abipolar mode using both the ring and tip electrodes of a lead or in aunipolar mode using only one of the electrodes of the lead with thedevice housing or can 60 serving as a ground electrode. As explainedbelow, one way in which the device may alter the spatial distribution ofpacing is to switch from unipolar to bipolar pacing (or vice-versa) orto interchange which electrodes of a bipolar lead are the cathode andanode during bipolar pacing. A shock pulse generator 50 is alsointerfaced to the controller for delivering a defibrillation shock via apair of shock electrodes 51 to the atria or ventricles upon detection ofa shockable tachyarrhythmia.

The controller 10 controls the overall operation of the device inaccordance with programmed instructions stored in memory, includingcontrolling the delivery of paces via the pacing channels, interpretingsense signals received from the sensing channels, and implementingtimers for defining escape intervals and sensory refractory periods. Anexertion level sensor 330 (e.g., an accelerometer, a minute ventilationsensor, or other sensor that measures a parameter related to metabolicdemand) enables the controller to adapt the pacing rate in accordancewith changes in the patient's physical activity. A telemetry interface40 is also provided which enables the controller to communicate with anexternal programmer.

In one embodiment, the exertion level sensor is a minute ventilationsensor which includes an exciter and an impedance measuring circuit. Theexciter supplies excitation current of a specified amplitude (e.g., as apulse waveform with constant amplitude) to excitation electrodes thatare disposed in the thorax. Voltage sense electrodes are disposed in aselected region of the thorax so that the potential difference betweenthe electrodes while excitation current is supplied is representative ofthe transthoracic impedance between the voltage sense electrodes. Theconductive housing or can may be used as one of the voltage senseelectrodes. The impedance measuring circuitry processes the voltagesense signal from the voltage sense electrodes to derive the impedancesignal. Further processing of the impedance signal allows the derivationof signal representing respiratory activity and/or cardiac blood volume,depending upon the location the voltage sense electrodes in the thorax.(See, e.g., U.S. Pat. Nos. 5,190,035 and 6,161,042, assigned to theassignee of the present invention and hereby incorporated by reference.)If the impedance signal is filtered to remove the respiratory component,the result is a signal that is representative of blood volume in theheart at any point in time, thus allowing the computation of strokevolume and, when combined with heart rate, computation of cardiacoutput.

The controller is capable of operating the device in a number ofprogrammed pacing modes which define how pulses are output in responseto sensed events and expiration of time intervals. Most pacemakers fortreating bradycardia are programmed to operate synchronously in aso-called demand mode where sensed cardiac events occurring within adefined interval either trigger or inhibit a pacing pulse. Inhibiteddemand pacing modes utilize escape intervals to control pacing inaccordance with sensed intrinsic activity such that a pacing pulse isdelivered to a heart chamber during a cardiac cycle only afterexpiration of a defined escape interval during which no intrinsic beatby the chamber is detected. Escape intervals for ventricular pacing canbe restarted by ventricular or atrial events, the latter allowing thepacing to track intrinsic atrial beats. Cardiac function therapy,whether for the purpose of cardiac resynchronization or for reversal ofremodeling, is most conveniently delivered in conjunction with abradycardia pacing mode where, for example, multiple excitatorystimulation pulses are delivered to multiple sites during a cardiaccycle in order to both pace the heart in accordance with a bradycardiamode and provide pre-excitation of selected sites.

A particular pacing mode for delivering cardiac function therapy,whether for stress reduction or resynchronization, includes a definedpulse output configuration and pulse output sequence, where the pulseoutput configuration specifies a specific subset of the availableelectrodes to be used for delivering pacing pulses and the pulse outputsequence specifies the timing relations between the pulses. The pulseoutput configuration is defined by the controller selecting particularpacing channels for use in outputting pacing pulses and by selectingparticular electrodes for use by the channel with switch matrix 70. Thepulse output configuration and sequence which optimally effects reverseremodeling by selectively reducing myocardial wall stress may or may notbe the optimum pulse output configuration and sequence for maximizinghemodynamic performance by resynchronizing ventricular contractions. Forexample, a more hemodynamically effective contraction may be obtained byexciting all areas of the myocardium simultaneously, which may noteffectively promote reversal of the hypertrophy or remodeling.

3. Demand-Based Cardiac Function Therapy

In order to deliver cardiac function therapy on a demand basis, thecontroller of an implantable cardiac device is programmed to suspenddelivery of the cardiac function therapy and assess the patient'scardiac function by means of one or more sensing modalities. A decisionalgorithm is then employed to determine subsequent therapy. In oneembodiment, a binary decision algorithm either continues or indefinitelysuspends the cardiac function therapy based upon the cardiac functionassessment. For example, stress reduction therapy may be employed in aheart failure or post-MI patient to effect reversal of cardiacremodeling. If the cardiac function assessment indicates that thepatient's condition is unchanged or has deteriorated, the device resumesthe stress reduction therapy. If the patient's cardiac function hasimproved sufficiently, on the other hand, the device indefinitelyterminates the therapy. Delivery of cardiac resynchronization therapymay similarly be continued or terminated based upon the cardiac functionassessment. After termination of cardiac function therapy, the devicemay continue to monitor the patient's cardiac function, periodically orotherwise, so that therapy can be resumed if needed. In addition, thedevice may periodically switch on therapies and see whether patientconditions have evolved gradually and therapy delivery is warrantedagain.

In another embodiment, the delivery of cardiac function therapy ismodified in accordance with the assessment of cardiac function. Thecardiac function therapy may be modified by changing the pulse outputconfiguration, the pulse output sequence, and/or various pacingparameters. For example, the device may change from a pulse outputconfiguration and sequence considered optimal for reversal of remodelingto one considered optimal for resynchronization pacing or vice-versa asa result of the cardiac function assessment. In another example, pacingparameters such as the length of one or more escape intervals, abiventricular offset interval for biventricular pacing, or an AV delayinterval for atrial tracking or AV sequential pacing are changed inaccordance with the cardiac function assessment.

a. Assessment of Cardiac Function

One means by which cardiac function may be assessed is by measuringcardiac output and comparing it with the patient's measured exertionlevel. As described earlier, cardiac output may be measured by animpedance technique in which transthoracic impedance is measured andused compute stroke volume. The stroke volume integrated over time (oraveraged and multiplied by heart rate) gives the patient's cardiacoutput. A look-up table or linear function may be used to compute whatcardiac output is considered adequate for a given exertion level. Basedupon these measurements, the device may then decide whether cardiacfunction therapy is warranted.

Another means for assessing cardiac function is by determining theautonomic balance of the patient. It is well-known that an increase inthe activity of the sympathetic nervous system may be indicative ofmetabolic stress and the need for increased cardiac output. One means bywhich increased sympathetic activity may be detected is via spectralanalysis of heart rate variability. Heart rate variability refers to thevariability of the time intervals between successive heart beats duringa sinus rhythm and is primarily due to the interaction between thesympathetic and parasympathetic arms of the autonomic nervous system.Spectral analysis of heart rate variability involves decomposing asignal representing successive beat-to-beat intervals into separatecomponents representing the amplitude of the signal at differentoscillation frequencies. It has been found that the amount of signalpower in a low frequency (LF) band ranging from 0.04 to 0.15 Hz isinfluenced by the levels of activity of both the sympathetic andparasympathetic nervous systems, while the amount of signal power in ahigh frequency band (HF) ranging from 0.15 to 0.40 Hz is primarily afunction of parasympathetic activity. The ratio of the signal powers,designated as the LF/HF ratio, is thus a good indicator of the state ofautonomic balance, with a high LF/HF ratio indicating increasedsympathetic activity. An LF/HF ratio which exceeds a specified thresholdvalue may be taken as an indicator that cardiac function is notadequate.

A cardiac rhythm management device can be programmed to determine theLF/HF ratio by analyzing data received from its ventricular sensingchannels. The intervals between successive ventricular senses, referredto as RR intervals, can be measured and collected for a period of timeor a specified number of beats. In order to derive a signal representingheart rate variability during a sinus rhythm, ectopic ventricular beats(i.e., premature ventricular contractions or PVCs) can be detected bymonitoring whether a P wave precedes each R wave, with the RR intervalsbefore and after the PVC changed to an interpolated or otherwisefiltered value. The resulting series of RR interval values is thenstored as a discrete signal. The signal can be used directly as indexedby heartbeat such that each value of the signal represents an RRinterval for a particular heartbeat. Preferably, however, the signal isresampled at a specified sampling frequency in order to equalize thetime intervals between signal values and thus convert the signal into adiscrete time signal, where the sampling frequency is selected to meetthe Nyquist criterion with respect to the frequencies of interest. Inany case, the RR interval signal can then be analyzed to determine itsenergies in the high and low frequency bands as described above.

Spectral analysis of an RR interval signal can be performed directly inthe frequency domain using discrete Fourier transform or autoregressiontechniques. Frequency domain analysis is computationally intensive,however, and may not be practical in an implantable device. Atime-domain technique for determining the high and low frequencycomponents of the signal is therefore preferably used. FIG. 2illustrates the functional components of an exemplary system for doingthis that can be implemented as code executed by the controller and/ordedicated hardware components. The RR interval signal obtained asdescribed above is input to both a low band digital filter 201 and ahigh band digital filter 202. The low band filter 201 is a bandpassfilter with a passband corresponding to the LF band (e.g., 0.04 to 0.15Hz), while the high band filter 202 is a bandpass filter with a passbandcorresponding to the HF band (e.g., 0.15 to 0.40 Hz). The outputs offilters 201 and 202 are then input to power detectors 203 and 204,respectively, in order to derive signals proportional to the power ofthe RR interval signal in each of the LF and HF bands. Power detectionmay be performed, for example, by squaring the amplitude of the signaland integrating over a specified average time. The output of powerdetector 203 is thus a signal P1 that represents the power of the RRinterval signal in the LF band, and the output of power detector 204 isa signal P2 representing the power in the HF band. The signals P1 and P2are next input to a divider 205 that computes the quantity S1/S2 whichequals the LF/HF ratio. The LF/HF ratio is then input to a movingaverage filter 206 that computes an average value for the ratio over aspecified period (e.g., 5 minutes). An updated LF/HF ratio may becomputed in this manner on a beat-to-beat basis.

In the above description, heart rate variability was derived from the RRinterval signal during normal sinus rhythm. It should also beappreciated that, if normal sinus rhythm is present, the RR interval isequivalent to the interval between successive atrial senses. As usedherein, therefore, the term RR interval should be regarded as theinterval between heart beats during sinus rhythm whether the beats areatrial or ventricular. Also, as an alternative to time-domain filtering,a statistical method of estimating the LF/HF ratio may be employed asdescribed in U.S. patent application Ser. No. 10/436,876 filed May 12,2003 and herein incorporated by reference.

Other means of assessing cardiac function may also be employed todeliver demand-based cardiac function therapy. The impedance techniquefor measuring cardiac output discussed above may also be used to measureventricular volumes at various stages of the cardiac cycle such asend-diastolic and end-systolic volumes and used to compute parametersreflective of cardiac function such as ejection fraction. Theimplantable device may also be equipped with other sensing modalitiessuch as a pressure transducer 85 shown in FIG. 1. Such a pressuretransducer may be attached to an intravascular lead and be appropriatelydisposed for measuring diastolic filling pressures and/or systolic pulsepressures.

b. Exemplary Algorithm

FIG. 3 illustrates an exemplary algorithm for demand-based cardiacfunction therapy as could be implemented by appropriate programming ofthe implantable device controller. Starting at step 200, the deviceinitiates cardiac function therapy such as stress reduction pacing orresynchronization pacing with a specified pacing mode and using aspecified pulse output configuration and pulse output sequence. At step201, either at periodic intervals or upon command from an externalprogrammer via the telemetry interface, the device suspends furtherdelivery of cardiac function therapy. At step 202, the device nextbegins an assessment of the patient's cardiac function using one or moresensing modalities. In this embodiment, the device computes cardiacoutput by measurement of the heart rate and cardiac stroke volume viathe intra-thoracic impedance method. The patient's exertion level 203(e.g., either activity level or minute ventilation) is then measured andcompared with the measured cardiac output measurement at step 203 todetermine whether the patient's cardiac output is adequate for thatparticular exertion level. Based upon this comparison a first cardiacfunction parameter CFP1 may be computed at step 204 which, if below aspecified threshold level, indicates inadequate cardiac function. Next,the patient's autonomic balance is assessed at step 205, and a secondcardiac function parameter CFP2 is computed which is indicative of theextent of metabolic stress experienced by the patient. This parametercan also be compared with a specified threshold value fordecision-making purposes. Based upon the computed cardiac functionparameters CFP1 and CFP2, the device at step 206 decides whether thepatient's cardiac function is inadequate. If so, the device continuescardiac function therapy by returning to step 200. If the computedcardiac function parameters indicate that the patient's cardiac functionhas improved to a sufficient extent, on the other hand, the deviceindefinitely suspends further delivery of cardiac function therapy atstep 207.

Although the invention has been described in conjunction with theforegoing specific embodiments, many alternatives, variations, andmodifications will be apparent to those of ordinary skill in the art.Such alternatives, variations, and modifications are intended to fallwithin the scope of the following appended claims.

1. An implantable device for delivering cardiac function therapy to apatient, comprising: sensing channels for sensing cardiac electricalactivity at a plurality of myocardial sites; pacing channels fordelivering pacing pulses to a plurality of myocardial sites; and, acontroller for controlling the delivery of pacing pulses in accordancewith a programmed pacing mode and with a defined pulse output sequenceand pulse output configuration for delivering cardiac function therapy;wherein the controller is programmed to temporarily suspend delivery ofcardiac function therapy, assess the patient's cardiac function while nocardiac function therapy is being delivered, and either re-initiate orcontinue the delivery of cardiac function therapy based upon the cardiacfunction assessment.
 2. The device of claim 1 wherein the cardiacfunction therapy is multi-site ventricular pacing which improves thepatient's cardiac pumping performance.
 3. The device of claim 1 whereinthe cardiac function therapy is multi-site ventricular pacing whichpre-excites selected myocardial regions in order to redistributemyocardial wall stress during systole for the purpose of reversingventricular remodeling.
 4. The device of claim 1 further comprising asensor for measuring cardiac output and wherein the cardiac functionassessment includes comparing the measured cardiac output to a specifiedthreshold value.
 5. The device of claim 4 wherein the cardiac outputsensor is a trans-throracic impedance measuring circuit.
 6. The deviceof claim 4 further comprising an exertion level sensor and wherein thecardiac function assessment includes comparing a function of themeasured cardiac output and measured exertion level to a specifiedthreshold value.
 7. The device of claim 1 wherein the cardiac functionassessment includes an assessment of the patient's autonomic balance bymeasuring the patient's heart rate variability.
 8. The device of claim 7further comprising: circuitry for measuring and collecting timeintervals between successive chamber senses and storing the collectedintervals as a discrete RR interval signal, filtering the RR intervalsignal into defined high and low frequency bands, and determining thesignal power of the RR interval signal in each of the low and highfrequency bands, referred to LF and HF, respectively; and, circuitry forcomputing an LF/HF ratio and assessing cardiac function by comparing theLF/HF ratio to a specified ratio threshold value.
 9. The device of claim1 wherein the controller is programmed to temporarily suspend deliveryof cardiac function therapy and assess the patient's cardiac functionupon a command from an external programmer.
 10. The device of claim 1wherein the controller is programmed to temporarily suspend delivery ofcardiac function therapy and assess the patient's cardiac function atperiodic intervals.
 11. A method for operating an implantable devicewhich delivers cardiac function therapy to a patient, comprising:delivering pacing pulses in accordance with a programmed pacing mode andwith a defined pulse output sequence and pulse output configuration fordelivering cardiac function therapy; and, temporarily suspendingdelivery of cardiac function therapy, assessing the patient's cardiacfunction while no cardiac function therapy is being delivered, andeither re-initiating or continuing the delivery of cardiac functiontherapy based upon the cardiac function assessment.
 12. The method ofclaim 11 wherein the cardiac function therapy is multi-site ventricularpacing which improves the patient's cardiac pumping performance.
 13. Themethod of claim 11 wherein the cardiac function therapy is multi-siteventricular pacing which pre-excites selected myocardial regions inorder to redistribute myocardial wall stress during systole for thepurpose of reversing ventricular remodeling.
 14. The method of claim 11further comprising measuring cardiac output and wherein the cardiacfunction assessment includes comparing the measured cardiac output to aspecified threshold value.
 15. The method of claim 11 wherein thecardiac output is measured by measuring a trans-throracic impedance andheart rate.
 16. The method of claim 14 further comprising measuring thepatient's exertion level and wherein the cardiac function assessmentincludes comparing a function of the measured cardiac output andmeasured exertion level to a specified threshold value.
 17. The methodof claim 11 wherein the cardiac function assessment includes anassessment of the patient's autonomic balance by measuring the patient'sheart rate variability.
 18. The method of claim 17 further comprising:measuring and collecting time intervals between successive chambersenses and storing the collected intervals as a discrete RR intervalsignal, filtering the RR interval signal into defined high and lowfrequency bands, and determining the signal power of the RR intervalsignal in each of the low and high frequency bands, referred to LF andHF, respectively; and, computing an LF/HF ratio and assessing cardiacfunction by comparing the LF/HF ratio to a specified ratio thresholdvalue.
 19. The method of claim 11 wherein the suspension of cardiacfunction therapy and assessment of the patient's cardiac function areperformed upon a command from an external programmer.
 20. The method ofclaim 11 wherein the suspension of cardiac function therapy andassessment of the patient's cardiac function are performed at periodicintervals.